The inkscape project is considering removing the flatpak format from the website's resources/downloads pages because of lack of maintenance. There is no maintainer of the flatpak/flathub format at ...
The Flatpak is already packaged and works well. It just needs to be maintained from a person that joins the Inkscape community.
This would allow further improvements like Portal support and making the app official on Flathub.
From the conversation it seems to be a similar situation to the project I'm with is in. The flatpak is essentially community maintained rather than being directly supported by the team. To become verified it needs to be done so by a representative of the maintainers of the software. To be verified it doesn't have to have a team member involved in it but this is a requirement Inkscape seem to have imposed.
For us we just aren't in a position to want to support it officially just yet, we have some major upgrades coming to our underlying tech stack that will introduce a whole bunch of stuff that will allow various XDG portals etc. to work properly with the Flatpak sandboxing model. To support it now would involve tons of workarounds which would need to be removed later.
Flatpak does verify the integrity of files as it is downloading/installing them. For ostree remotes this is done using GPG signatures (which are better than mere checksums). If you want to see the commit ID (which is like a checksum) for something on flathub use e.g. flatpak remote-info -c flathub org.gnome.Builder and for the local copy flatpak info -c org.gnome.Builder. For OCI remotes we at least check SHA256 sums and there might be more integrity verification mechanisms I'm unaware of.